This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Epidemiological studies have indicated that low calcium and high oxalate intakes are risk factors for the formation of calcium oxalate kidney stones. Oxalate normally complexes with calcium in the intestine forming insoluble cyrstals which are eliminated in the fecal stream. We have shown using diets controlled in oxalate content that urinary oxalate excretion is proportional to the amount of oxalate ingested which is directly correlated with the risk of stone formation. Oxalobacter formigenes has been identified as the predominant organism degrading oxalate in human feces so colonization with this organism has been hypothesized to limit stone risk. This non-pathogenic, anaerobic bacterium uses oxalate as its main carbon source, converting it to formate and carbon dioxide. Several studies have suggested that calcium oxalate stone formers are less likely to have OxF than non-stone formers. Most dietary studies to date have used a constant ratio of calcium to oxalate in meals. This may not truly represent the normal dietary patterns in most of the U.S. population. We propose that maintaining an optimal balance between calcium and oxalate intake with each meal may limit transient episodes of increased oxalate excretion and thus reduce stone risk. We further propose that such transient increases in urinary oxalate excretion are greater in non-colonized individuals. This study will compare the response of colonized and non-colonized individuals to diets balanced and imbalanced in their calcium to oxalate ratios.